Method of and apparatus for making deposited carbon resistors



D. E. JOHNSON ETAL 3,105,288

METHOD OF AND APPARATUS FOR MAKING DEROSITEIS CARBON RESISTORS Filed Feb. 27, 1959 2 DONALD EJOHNSON BY JAMES L. OWENS l LENGTH OF SPIRAL IN INCHES ATTORNEY Oct. 1-, 1963 a H I m W o m E A 6 m M 3 I. \l 2 QE 4 5 .mm U L 4 I 4 nvus P. 2 1 2 1 M 5 A 9 c. D. 3 8 l 22 H 2 7 M m 3 u n vE 2 O m Q w T 12 4 EJ 2 3 4 RE 23 K I R I d 3 3 2H m I m 4\ 4 J. A H m 2 a I n 5 m A u m PD u I R m E mm m w h Rm m z 6 L OT 7 M I l n A .R, .E M 5\ .Mv n U -fi v 4 m 4 6210 mus: uuzfimaum United States Patent METHGD 0F AND APPARATUS FOR MAKING DEPOSITED (IARBON RESHSTORS Donald E. Johnson, Tobaccovilie, and James L. Owens,

Winston-Salem, NiC assignors to Western Electric (Iompany, incorporated, New York, N.Y., a corporation of New York Filed Feb. 27, 1959, Ser. No. 796,143

6 (Iiaims. (Ci. 29-15562) This invention relates to a method of and apparatus for making articles having desired surface film characteristics and more particularly to a method of and apparatus -for making deposited carbon resistors wherein any deposited carbon resistor film irregularity is sensed.

=In the manufacture of electrical resistors such as deposited carbon resistors, it has been customary to provide a cylindrical insulating base having a film of carbon deposited thereon. A helical grove is progressively formed through the carbon film from one end of the insulating base toward the other. While the groove is being formed, the resistance of the resistor is measured, and when the resistance meets the required value, the groove forming operation is stopped. However, this operation for grooving resistors to value does not include a method of or apparatus for sensing film irregularities as the grove is formed. Hence, resistors manufactured by known techniques may have unevenly coated or chipped bases which would not be detected during the resistor grooving. Resistors with such defects develop hot spots which eventually cause burn outs" in. the resistor film.

The present invention provides a method of and apparatus for making electrical resistors wherein any irregularity in resistance is detected by sensing the change of resistance per unit length of resistor i.e., the rate at which the resistance of the film changes, during a grooving-to-value operation. If such change is constant and other variables such as temperature are maintained constant, the resistance film is uniform. Conversely, if such change is not constant, other variables being maintained constant, the resistance film is non-uniform and the resistor should be rejected as defective.

An object of the present invention is the provision of a method of and apparatus for making articles having desired surface film characteristics.

Another object of the invention is the provision of a method of and apparatus formakin-g deposited carbon resistors.

Another object is to provide a method of and apparatus for making deposited carbon resistors wherein any deposited carbon film irregularity is sensed.

A method of detecting resistance film irregularities of a deposited carbon resistor illustrating certain features of the invention may include the steps of cutting a helical groove of constant pitch through the resistance fi'lrn to form a resistance path, and simultaneously sensing by means of a difi erentiating network the time-rate change of resistance of the resistance path as it is progressively formed.

Apparatus illustrating certain features-of the invention may include means for gripping and rotating a deposited carbon resistor and means for cutting a helical groove of constant pitch through the deposited carbon film as the resistor is rotated, thereby forming a resistance path of a predetermined resistance value. As the helical groove is progressively cut, a differentiating network connected electrically across the resistor simultaneously senses the change of resistance per length of resistance path i.e., senses time-rate change of the resistance path. it such change is substantially constant, there is no output signal ttrom the difierentiating network. On

the other hand, if such change is not constant or erratic, there is an output signal from the differentiating network indicating that the resistor being grooved has the carbon deposited irregularly thereon. The output signal is used to energize a reject device whereupon the faulty resistor is removed.

A complete understanding of the invention may be had from the following detailed description of a specific embodiment thereof when read in conjunction with the appended drawings, in which:

FIG. 1 is a front elevational view of a preferred embodiment of the invention;

FIG. 2 illustrates a sectional view of apparatus taken along the line 2-2 of FIG. 1;

FIG. 3 shows a deposited carbon resistor which has been grooved to value; and

FIG. 4 discloses a graph of a resistace versus length curve of a resistor having been helically grooved to a predetermined value.

Referring now to the drawing, wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in FIGS. 1 and 3 a resistor blank 11 having a cylindrical base of insulating material, such as ceramic or the like, with a deposit thereon of a resistance material possessing resistance characteristics desired in the finished resistor. This resistance coating may be carbon deposited on the insulating base by any one of several conventional depositing methods, such as heating the insulating base in a controlled atmosphere of methane.

The apparatus which comprises a portion of the preferred embodiment of the invention substantially being symmetrical, only the left side of the apparatus as viewed in FIG. 1 need be described hereinbelow. The resistor blank 11 is held securely :for rotation by a chuck 12, FIG. 1, coupled fixedly to one end of a first independent drive shaft 13, the other end of which is connected to a first universal joint .14. The universal joint 14 is fixedly engaged to a telescopic shaft 15, adapted for transverse movement, and in turn the shaft is coupled to a second universal joint 16. A second independent drive shaft 17 is coupled to the joint 16 and is driven by a motor means 18. The motor means includes a gear arrangement, not shown, which transversely travels along splines, not shown, causing arm :19 to move left or right along a splined guide rod 21 to obtain transverse motion of the resistor blank 11 with respect to the fixed position of a cutting wheel 22.

As shown in FIGS. 1 and 2, each first independent drive shaft 13 is held for rotational and axial movement by arm 19 which is held pivotally by the splined guide rod 21, the rod being rotatably supported by a pair of bearing members 23 fixed to a frame 24. The arms 19 are counterbalanced to allow the resistor blank 11 to move pivotally with a predetermined pressure against the cutting wheel 22. The first independent drive shafts 13 are axially moved by the arms 19 in cooperation with the gear arrangement, not shown, of the independent motor means 18 so as to transmit such movement to the resistor blank 11 and cause it to move transversely with respect to the cutting wheel 22, which is rotated in stationary position. Also, horizontal inward and outward movement of the chucks 12 for engagement and disengagement of the blank 11 is provided by the arms 19 through a link connector generally designated by the numeral 25 having an arm 26, a link 27 pivotally connected to one end of the arm 26, and another link 28 pivotally connected to the other end of the arm 26. The link connector 25 is rotated about a pivot 29 by means not shown.

The pivotal movement of the arms 19 about pivot 31, FIG. 2, is controlled by a solenoid 32 fixed to the frame 24 and having an armature 33 connected pivotally to a lever arm 34. The lever arm 34is coupled to the splined guide rod 21 to impart rotation to the rod 21 upon vertical movement of the armature 33. Such rotational movement of the splined guide rod 21 is transmitted to amplifier 38, a normally closed relay 39, a voltage source 41 (for the solenoid 32, a conventional differentiating network 42 (composed of a simple circuit including resistor 55 and capacitor 56), a conventional amplifier 43, a resistor rejection device 44, and a recorder 45 (optional).

Operation 'The resistor blank Ill to be helically grooved to value and tested for film irregularities is positioned within the chucks 12 by actuating a rotating means, not shown, to

rotate the armlt of the link [connector in a clockwise direction about the pivot 29 thus moving horizontally the arms 19 and the chucks 12' away from each other. While the chucks 12 are in this position, the resistor blank l ll is centered within one of the chucks l2 and the rotating means is again actuated to rotate the arm 26 of the link connector 25 in a counterclockwise direction to move the arms ll9 along their horizontal axes, and the chucks 12 toward each other so that both chucks l2 grip the resistor blank 11 for rotation.

After the resistor blank 11 is positioned within the chucks 12;, one end of the resistor blank 11 is allowed to rest against the cutting wheel 22, as shown in cross section in FIG. 2. As before mentioned, the arms 19 are counterbalanced to allow the resistor blank to rest pivotally against the cutting wheel 22 with a predetermined pressure. Before the helical grooving is begun, control means, not shown, guide the second independent driving shafts 17 such that the left end of the resistor blank 11 rests against the cutting wheel 22. At this time the left telescopic shaft 115 is axially elongated and the right telescopic shaft 15 is foreshortened.

When the resistor blank 11 is positioned within the chucks 12, it becomes one branch of the Wheatstone bridge '37 due to the electrical connections from the Wheatstone bridge 37, through the slip rings 35, the first driving shafts 13, the chucks l2, and finally through the resistor'blank 11. The arms 19 are appropriately insulated from shafts 13 so as not to short-circuit the resistor blank 11.

The resistor blank 11 causes a bridge unbalance and a consequent voltage across the center branch of the bridge which is amplified by the DC. amplifier 38 thereby im pressing a voltage across the normally closed relay 39.

The impressed voltage opens the contacts of the relay 39 to prevent any voltage fro-m the source 41 from being impressed across the solenoid 32, thus maintaining it inoperative. When the solenoid 32 is inoperative, the resistor blank 11 rests against the cutting wheel 22, as shown in section in FIG. 2.

- With the second independent driving shafts 17 maintaining one end of the resistor blank 11 against the cutting wheel 22, the helical grooving operation is begun by simultaneously actuating the respective independent motor means 18 to rotate the resistor blank 11 at a constant rate, and by actuating driving means, not shown, to rotate the cutting wheel 22 at, a constant rate. 7 Thus a helical groove 46 is progressively cut at a constant rate through the resistance film deposited on the base of resistor blank 11. As the groove 46 is progressively cut from one end of the resistor blank 11 to the other, the electrical resistance of the resistor blank t]. increases until. the Wheatstone bridge 37 is in balance, at which time the bridge output falls to zero. With zero output from the bridge 37, the normally closed relay 3? closes its contacts. When these relay contacts are closed, a voltage from the source 41 is impressed across the solenoid 32 to cause the armature 33 (FIG. 2) to move upwardly. Hence, the lever arm 34, the arms 19, the first independent driving shafts 13, the chucks 12, and the resistor blank 11 are pivoted counterclockwise, as shown by the arrow of FIG. 2, away fro-m the cutting wheel 22.

While the resistor blank 11 is being grooved to value, the ratio of resistance to length of the resistor blank 11 changes. If the deposited film of the resistor blank 11 is uniform and other variables such as the pitch of the helical groove 46, the temperature of the resistor blank 11, and the rate at which the grooves 46 is formed are constant, such change in resistance per length will be constant, i.e., linear. On the other hand, if the deposited film of the resistor blank 11 is non-uniformand the other variables mentioned above are constant, the. change of resistance per length or the change of resistance per time will not be constant, i.e., non-linear; Such unconstant change will be detected by the differentiating network 42 which is connected across the resistor blank '11 through the Wheatstone bridge 37. When the differentiating network 42 detects the uncons'tant change, a signal is produced and carried to the amplifier 43, which in turn sends a signal to the resistor rejection device 44, causing the defective resistor blank to be removed from the grooving and testing apparatus. 7 As an optional feature of this invention, a recorder 45 may be connected across the output of the amplifier 43 to make a record of resistors having non-uniform or irregular deposited films.

In FIG. 4 there is shown a graph in rectangular coordinates of a resistance versus length curve of the resistor blank 11 being grooved to value. As the resistor blank 11 is grooved from one end toward the other, if the deposited film is uniform the change of resistance per length of resistor blank 11 is constant and is represented on the graph by a solid straight line 47. On the other hand if the deposited film-is non-uniform, the change of resistance per length is not constant and this change is versus length curve of the resistor blank 11.

It should be understood that the above-described embodiment of the invention is merely illustrative and that numerous modifications may be madewithin the spirit and scope of the invention. Further, the particular apparatus illustrated is only one example of several types of apparatus which may be included in practicing the invention and the invention is not limited solely to the useof this apparatus.

What is claimed is:

1. A method of detecting film irregularities on deposited carbon resistors having an insulating base with a thin film of carbon deposited thereon, which comprises the steps of removing a portion of the film at a constant rate so as to form a parallel spiral resistance path ex tending helically around the base, and simultaneously, differentiating the time-rate change of resistance of the path as it is formed, said dilferentiatedchange being an indication of the fihn irregularity, and rejecting any resistor for which the differentiated change deviates from predetermined limits.

2. A method of making electrical resistors which comprises the steps of forming a resistance path on an insulating base at a constant rate, continuously measuring the cumulative resistance value of the path as it is formed, simultaneously differentiating the time rate change of resistance of the resistance path as it is formed, and rejecting any resistor when the differentiated change deviates from predetermined limits.

3. An apparatus :fior detecting irregularities on deposited carbon resistors comprising means for gripping and for rotating the resistor at a uniform rate, means for progressively cutting a helical groove of constant pitch through the carbon film as the resistor is rotated to form a path of predetermined resistance value at a constant rate, a differentiating network electrically connected across the resistor for simultaneously sensing the timerate change of resistance of the resistance path as it is progressively formed, and means responsive to said change for rejecting a resistor having a non-uniform film.

4. Apparatus for detecting coating irregularities on electrical resistors comprising means for forming a resistance path at a constant rate on an insulating base to a predetermined value, means electrically connected across the resistor for continuously measuring the cumulative resistance value of the path as it is progressively formed, a differentiating network electrically connected across the resistor for simultaneously sensing the time-rate change of resistance of the resistance path as it is progressively formed, and means responsive to said change for rejecting a resistor having an irregular resistance path.

'5. In an apparatus for cutting a resistance path in carbon deposited on a ceramic core, a pair of chucks for holding a core, means for moving the chucks toward and away from each other to hold and release the core, means for rotating the chucks, a cutting means, means for imparting relative movement between said cutting means and chucks for cutting a spiral path in said deposited carbon, means for measuring the resistance value of the cut spiral path of deposited carbon, a difierentiating circuit connected to said measuring means and responsive to time-rate changes in resistance value exceeding a predetermined rate for generating an output signal, and means responsive to said output signal for removing a resistor from the chucks and the means for measuring resistance.

6. An apparatus for detecting irregularities in a film of carbon deposited on a hollow ceramic core of a deposited carbon resistor blank being grooved to value comprising a pair of chucks for gripping the resistor blank, means for rotating the chucks in unison to rotate the resistor blank at a constant rate, means for axially moving the chucks apart and together so that the resistor blank is gripped by the chucks, a cutting Wheel having its rotational axis parallel to the rotational axis of the resistor blank, the cutting wheel being axially stationary and having its cutting surface in removable contact with the resistor blank, means for rotating the cutting wheel at a constant rate, means for axially moving at a constant rate the rotating resistor blank across the rotating cutting wheel to form a helix of constant pitch through the carbon film, means for insulating the moving means such that the resistor blank is not short-circuited, a pair of slip rings respectively fixed to the chuck rotating means, a differentiating network coupled to the slip rings for sensing the rate at which the resistance increases as the helix is formed in the carbon film, and means responsive to the output of said differentiating network sensing an unconstant rate for indicating an irregularity in the film of carbon deposited on the hollow ceramic core of the deposited carbon resistor blank and rejecting any blank having an irregularity in the film of carbon thereon.

References Cited in the file of this patent UNITED STATES PATENTS 1,256,599 Schoop Feb. 19, 1918 1,859,112 Silbcrstein May 17, 1932 2,077,187 Richter Apr. 13, 1937 2,273,941 Dorn Feb. 24, 1942 2,330,782 Morelock Sept. 28, 1943 2,418,804 Hood Apr. 8, 1947 2,500,605 de Lange et a1. Mar. 14, 1950 2,503,418 Scranton Apr. 11, 1950 2,545,576 Godley Mar. 20, 1951 2,595,189 Dewan Apr. 29, 1952 2,792,620 Kohring May 21, 1957 FOREIGN PATENTS 747,257 Germany Sept. 18, 1944 

3. AN APPARATUS FOR DETECTING IRREGULARITIES ON DEPOSITED CARBON RESISTORS COMPRISING MEANS FOR GRIPPING AND FOR ROTATING THE RESISTOR AT A UNIFORM RATE, MEANS FOR PROGRESSIVELY CUTTING A HELICAL GROOVE OF CONSTANT PITCH THROUGH THE CARBON FILM AS THE RESISTOR IS ROTATED TO FORM A PATH OF PREDETERMINED RESISTANCE VALUE AT A CONSTANT RATE, A DIFFERENTIATING NETWORK ELECTRICALLY CONNECTED ACROSS THE RESISTOR FOR SIMULTANEOUSLY SENSING THE TIMERATE CHANGE OF RESISTANCE OF THE RESISTANCE PATH AS IT IS PROGRESSIVELY FORMED, AND MEANS RESPONSIVE TO SAID CHANGE FOR REJECTING A RESISTOR HAVING A NON-UNIFORM FILM. 